Equalization is the process of removing (transmitter and) channel-induced distortion from a received signal.
In cellular radiotelephone systems, ambulatory (mobile) radiotelephones are Automatically Frequency Controlled (AFC) to the carrier frequency of the fixed transmitter with which it is communicating (or monitoring). (Fixed) scan receivers are employed in adjacent cells to intermittently and rapidly acquire and monitor those mobile transmissions to determine which fixed transmitter would best serve that mobile (e.g., handoff). Since scan receives are simple monitoring apparatus (and do not actively carry on voice communications), they are less complex and may not have AFC. The problem then for scan receivers is that the cumulative effects of frequency stability and tolerance of crystal oscillators in the fixed transmitters, the mobile's AFC resolution (.+-.200 Hz), and the Doppler effect of ambulatory radiotelephones total in excess of 780 Hz of frequency offset error (distortion) on a 900 MHz carrier.
Typically, the monitored signal is acquired by looking for correlation against a known transmitted synchronization pattern, but this frequency distortion lessens the chances of detection of the sync word by complex correlation. Where the theoretical normalized vector sum of a quadrature modulated (pi/4 QPSK) signal in the absence of frequency offset error would have a perfect, 100% complex correlation (magnitude one, phase zero--10.angle.0), a 780 Hz frequency offset results in the summation of a curved vector path and a normalized correlation of only 66% (i.e., 0.66.angle.81). Clearly, some form of equalization is required to avoid a missed detection due to frequency-offset-induced low correlation.
This invention takes as its object to overcome these shortcomings and to realize certain advantages presented below.